Add Bazel verify-release-build job (#17705)

## Why

`main` recently needed
[#17691](https://github.com/openai/codex/pull/17691) because code behind
`cfg(not(debug_assertions))` was not being compiled by the Bazel PR
workflow. Our existing CI only built the fast/debug configuration, so
PRs could stay green while release-only Rust code still failed to
compile. This PR adds a release-style compile check that is cheap enough
to run on every PR.

## What Changed

- Added a `verify-release-build` job to `.github/workflows/bazel.yml`.
- Represented each supported OS once in that job's matrix: x64 Linux,
arm64 macOS, and x64 Windows.
- Kept the build close to fastbuild cost by using
`--compilation_mode=fastbuild` while forcing Rust to compile with
`-Cdebug-assertions=no`, which makes `cfg(not(debug_assertions))` true
without also turning on release optimizations or debug-info generation.
- Added comments in `.github/workflows/bazel.yml` and
`scripts/list-bazel-release-targets.sh` to make the job's intent and
target scope explicit.
- Restored the Bazel repository cache save behavior to run after every
non-cancelled job, matching
[#16926](https://github.com/openai/codex/pull/16926), and removed the
now-unused `repository-cache-hit` output from `prepare-bazel-ci`.
- Reused the shared `prepare-bazel-ci` action from the parent PR so the
new job does not duplicate Bazel setup boilerplate.

## Verification

- Used `bazel aquery` on `//codex-rs/tui:codex-tui` to confirm the Rust
compile still uses `opt-level=0` and `debuginfo=0` while passing
`-Cdebug-assertions=no`.
- Parsed `.github/workflows/bazel.yml` as YAML locally.
- Ran `bash -n scripts/list-bazel-release-targets.sh`.

scripts/list-bazel-release-targets.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "${repo_root}"
+
+# Keep this list focused on first-party Rust targets whose compile surface can
+# differ when `cfg(not(debug_assertions))` becomes active.
+#
+# Exclude the experimental `v8-poc` target because it pulls in expensive V8
+# build machinery that is unrelated to the release-only Rust regression this
+# workflow is meant to catch.
+printf '%s\n' \
+  "//codex-rs/..." \
+  "-//codex-rs/v8-poc:all"